Conventional hammer drills operate in two modes, namely, a hammer drill mode and a drill mode. In the hammer drill mode, a hammer bit is rotatably driven and axially reciprocated to drill holes in hard brittle materials such as a brick, mortar and concrete. In the drill mode, a drill bit is rotatably driven only to drill holes in softer less brittle materials such as wood and metal. To axially reciprocate and rotate the hammer bit in the hammer drill mode, the hammer drill contains a forwardly spring biased output spindle and a normally disengaged hammer clutch consisting of an input clutch plate and an output clutch plate. The plates have mutually opposed sets of teeth to axially reciprocate the spindle when the clutch is engaged. The output clutch plate is axially and rotatably fixed to the output spindle. The input clutch plate is axially and nonrotatably fixed in the housing and is engageable by the output clutch plate when an operator applies a rearward bias to the output spindle when engaging a hammer bit with a workpiece. When the output clutch plate is rotatably driven through a gear train, the output clutch plate is axially reciprocated by the ratcheting of the output clutch plate teeth over the fixed teeth of the input clutch plate. In the drill mode, the output spindle is locked in the forwardly spring biased position. And, the input and output clutch plates are fixed in a disengaged position regardless of the rearward biased applied to the output spindle by an operator. As the result, the output spindle is rotatably driven only.
Housings for such hammer drills are generally of two types. One type is a clam shell housing comprising two clam shell housing halves joined generally along an interface lying in a plane parallel to the axis of the output spindle. A second housing type is a jam pot housing comprising two housing halves joined along an interface lying generally in a plane perpendicular to the output spindle. For manufacture of a tool with a clam shell housing, the internal components are loaded into one clam shell half and then the second clam shell half is mounted over the components and first clam shell half. For manufacture of a tool with a jam pot housing, the components are end loaded into the front and rear housing portions which are generally barrel shaped. Then the two housing halves and the components assembled into each are attached together. Tools with a clam shell housing are generally considered to be lower in cost and easier to assemble than tools with a jam pot housing. Clam shell housings have, therefore, become widely used for high volume mass produced portable power tools.
In prior art clam shell hammer drills, it has been found that the durability of the hammering clutch is poor. And, it is therefore desirable to develop an improved, low cost hammer drill with a more durable hammer clutch.